1. Field of Invention
This invention relates generally to filterless amplifiers, and more particularly, to a double reference waves filterless modulation scheme for filterless amplifiers and method thereof to reduce EMI (Electronic-Magnetic Interference) and common mode noise.
2. Description of Related Art
Class-D audio power amplifiers are two to five time more efficient than class-AB audio power amplifiers. Because of their greater efficiency, class-D amplifiers require smaller power supplies and eliminate heat sinks, significantly reducing overall system costs, size and weight.
Class D audio power amplifiers convert audio signals into high-frequency pulses that switch the output in accordance with the audio input signal. Some class D amplifier use pulse width modulators (PWM) for generating a series of conditioning pulses that vary in width with the audio signal's amplitude. The varying-width pulses switch the power-output transistors at a fixed frequency. Other class D amplifiers may rely upon other types of pulse modulators. For heuristic purposes, the following discussion mainly refers to pulse width modulators, but those skilled in the art will recognize that class D amplifiers may be configured with other types of modulators.
FIG. 1 illustrates a Class D amplifier familiar to those skilled in the art. The input audio signal is input into an input amplifier 101 via capacitors CIN1 and CIN2. Then, a comparator 105 compares the amplifier's output at a negative input and the triangular waves, generated from triangular wave generator 103, at a positive input for generating PWM signals. The PWM signals are input into a gate drive 107 for driving transistors Q11˜Q14. Differential outputs, OUTP and OUTN, of the class D amplifier are respectively fed into low-pass filters (formed of an inductor L1 and a capacitor C1, and an inductor L2 and a capacitor C2) that convert the pulses back into an amplified audio signal for driving one or more loads 109 (for example, audio speakers).
The traditional class D amplifiers have differential outputs (OUTP and OUTN). Differential outputs are complementary to each other and include a swing range from ground to VDD.
FIGS. 2A˜2C illustrate the waveforms when the input signal is large, small and zero, respectively. In FIG. 2C, large output ripple currents are caused. For reducing the large output ripple currents, a large LC filter is needed, as shown in FIG. 1.
When the input signal is zero or very small, the traditional class D amplifier has a differential voltage across load with almost 50% duty cycle. This 50% duty cycle sinks and sources large currents through the load (the speaker), resulting additional power consumption.
Class-D amplifiers are not popular in many portable products because the traditional class-D amplifier requires large LC filter, which increases size and solution cost. Filterless class-D amplifiers eliminate the output filter while keeping the efficiency benefit. The filterless modulation scheme brings class-D amplifiers approximately equal to class-AB amplifiers in cost and size, but still gains the great efficiency advantages. A traditional filterless class D amplifier is shown in FIG. 3. FIGS. 4A and 4B illustrate waveforms of FIG. 3 when the input signal is large and small, respectively.
As shown in FIG. 3, the filterless class D amplifier for driving a load 315 at least includes capacitors CIN3, C31 and C32, resistors RS1˜RS4 and RFB31˜RFB34, a differential input amplifier 301, two integrators 305 and 307, a triangular wave generator 303, two comparators 309 and 311, a control logic & gate drive 313 and switching transistors Q31˜Q34. This modulation scheme reduces EMI and does not have low-pass filters (that is why it is called “filterless”). However, this kind of modulation scheme still has a problem relying on common mode voltage variation, since OUTP and OUTN are high or low at the same time, as shown in FIGS. 4A and 4B. This is equivalent to a large swing of common-mode voltage. The ternary modulation scheme suffers from high common mode switching noises and requires additional filter to suppress the common mode switching noises. The common mode switching noises are more severe when the input signal is small. Besides, this modulation scheme uses two integrators, which increases quiescent currents, circuit size and solution cost.
Therefore, a new and highly efficient class D audio power amplifier is needed to overcome drawbacks in the known art.